Certain B lymphoma and B hybrid cell lines can be induced to differentiate by anti immunoglobulin or LPS. LPS containing bacterial peptides is more effective than phenol-prepared LPS (Phe-LPS) in inducing terminal growth, but in contrast to Phe-LPS is a poor inducer of Ig secretion. Fetal calf serum contains factors which in sufficient amounts blocks this inductive effect. Medium conditioned by growth of hybrid TH 2.2 has a similar effect. Cytokines and lymphokines have pleiotropic effects on the growth and differentiation of different cell lines; eg. GM-CSF (perhaps an autocrine factor) has no effect on growth of line TH 2.2, but inhibits line F6.14.7. GM-CSF further inhibits TH 2.2 cells induced with LPS, but enhances the growth of F6.14.7 cells so induced. IL-4 has no effect on TH 2.2. It inhibits growth of uninduced F6.14.7, greatly enhances growth of this line induced with LPS, but has little effect on the line induced with DXS. Small purified murine B cells from spleen are induced by anti IgD and anti IgM coupled to Sepharose beads or high molecular weight dextran to secrete GM-CSF and a small amount of IL-3. The lymphokine activities were assayed on responsive cell lines, and each was specifically blocked by an appropriate monoclonal antibody.